Caption: Human red blood cells in hypertonic solution (1.01% NaCl), coloured scanning electron micrograph (SEM). Most cells appear shrunken or crenate and less biconcave than normal red blood cells. Red blood cells (RBCs), or erythrocytes, are the most common type of blood cell in vertebrates. RBCs take up oxygen in the lungs or gills and release it into tissues while squeezing through the body's capillaries. The cytoplasm of RBCs is rich in haemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the red color of the cells. The cell membrane is composed of proteins and lipids and this structure provides properties essential for physiological cell function (deformability and stability) while traversing the circulatory system, especially the capillary network. In humans, mature RBCs are flexible and oval biconcave disks. Magnification: x1,000 when shortest axis printed at 25 millimetres.